Civil wars don't just start. There's a build up. Republicans will begin to engage in acts of domestic terrorism like bombings and blowing up substations. Dems will definitely retaliate and that's how we get a civil war.
I doubt even this would cause a civil war. By world standards, U.S. citizens have pretty good lives, so most people wouldn't risk it. An unprecedented and complete collapse of the U.S. quality of life would be necessary. People would need to feel like they had nothing to lose, and much to gain to attempt to fight the U.S.
I suppose a quick coup is possible. If the DoD/military backed it, I'd guess most people would just accept it as the way things are, or try to emigrate to other countries.
The U.S., the "west", and possibly the entire world is probably just destined for a long slow decline as global capitalism eats itself. Probably transitioning to something worse featuring mass surveillance, disinformation like we've never seen (powered by "AI" tech), and even greater inequality. But I'm a pessimist :)
If by "retaliate", you mean arrest people, then yes. The closest dems get to doing things outside the law is black people "getting uppity" and accidentally setting fire to the local buger king.
What are you on about? Crazy cultist fraction of a political party act out in violence and the other political party decides regular authorities nor even military are not enough to handle organized domestic terrorism but retaliates by own volition? How? By blowing up the neighbouring substation? Political activists driving around in armoured gas powered SUVs vs electric vehicles competing who can damage society most? The absolute majority of people that just want a stable life and feel generally disconnected from politics goes bananas and start shooting and looting and killing over minor disagreements? Bitcoin and gold becomes the most valuable currency along with ammunition and canned beans? Or the worst crime against humanity and allow the normal course of legal process as if the leadership and participants were just like any other citizens doing unlawful activities? Oh no, where be my pearls?
I don't think Nancy pelosi is going to take to the streets with an RPG. I think individual people who identify as Democrat will take it upon themselves to retaliate.
The GOP is a party of cowards. They go along with Trump and all the crazy narratives only because they're afraid of being primaried and losing their positions.
There is a potential for violence and domestic terrorism but it would be a disorganized mess. Just the few true believers duped by the crazy narratives into doing something stupid.